Rough cast metal workpieces, such as crankshaft castings, must be machined down from their initially rough surface to produce smooth, accurately dimensioned surfaces, such as the bearing main journals. One common machining method is known as plunge turning, in which a single cutting tool is pushed, or "plunged", into the rough surface as the workpiece is rapidly rotated about its axis, removing the excess surface material in one continuous layer. Another method is the so called turn broaching method. Turn broaching is so called because the cutting tool used is a broach, rather than a single tool, and has a series of cutting teeth. The turn broach is moved relative to the rapidly rotating workpiece in such a way that the cutting teeth are incrementally advanced toward and into the rough surface, with the excess material being removed in progressive layers. While turn broaching has proved successful in producing surfaces of the desired quality, and is potentially more economical than plunge turning, tool life and productivity have still not been as great as had been initially hoped.
A typical turn broaching station is illustrated in FIG. 1. A suitable power source 10 rotates a crankshaft rough casting 12 while a turn broach 14 is advanced along a straight path perpendicularly to the axis of rotation. This incrementally advances a series of cutting teeth 16 into the rough outer surface of the casting 12. Excess material is thus progressively nibbled away by the advancing teeth 16. A typical turn broach 14, as shown in FIG. 2, generally consists of a cutter body 18 upon which the teeth 16 are removably mounted, so that they may be easily sharpened or replaced, with wear or breakage. Often, the cutting teeth will be divided up into various discrete sections or groupings, a "bumper" section that sees the workpiece surface first and is designed to remove casting scale, a "rougher" section that sees the workpiece next and removes the bulk of the metal, and a final "finisher" section that brings the machined surface within final specification. The prior art turn broach 14 does not include a bumper or finisher as such, but corresponds to a typical rougher section. Breakage and tool life problems, not surprisingly, are noticed most frequently in the rougher sections, where most of the work is done. A conventional turn broach typically has cutting teeth that are arranged linearly, that is, each tooth is incrementally advanced a uniform amount higher than the prior tooth, with the sum of all tooth increments equaling the total thickness of metal that it is necessary to remove. This incremental advance, at least in the straight line type of turn broach shown in FIG. 2, is generally referred to as the tool rise. To give a specific example, if a ten millimeter total thickness of excess workpiece metal must be removed, and there are ten teeth, then the first tooth is set to take a one millimeter initial depth of cut, and each of the next nine teeth is set one millimeter higher than the prior tooth, so as to machine a one millimeter thick layer each, at least in theory. Thus, the conventional tool rise takes into account only the total thickness of material to be removed, and just divides it up evenly over all the teeth.
A turn broach need not have the teeth arranged in a straight line as such, as in FIG. 2. For example, the cutter body could be arcuate in shape, with the teeth arranged equally angularly spaced, but located in a spiral about an axis of the cutter body. The incremental advance of each tooth, then, is not a tool rise as in the FIG. 2 broach, but is instead a progressively greater radius, measured relative to the cutter body axis. The arcuate turn broach is operated with its axis parallel to the axis of the rotating workpiece, and is then turned about its axis, rather than being advanced in a straight line. The effect is the same, however, which is that the teeth are incrementally advanced into the workpiece surface to progressively machine it down. The basic tooth configuration for the arcuate broach is really the same as the straight line broach. That is, the incremental advance in radius from tooth to tooth is still linear and uniform, and is calculated only on the basis of the total thickness of metal to be removed divided over the number of teeth.
The conventional uniform or linear tool rise described above is often referred to in prior art references in general terms as a "progressive" tooth rise. This conventional cutting tooth configuration has apparently been used almost universally, without questioning its efficiency. The prior art has recognized that as a tool cuts, there are deflections induced in the workpiece, which deflections can cause the workpiece to oscillate and bounce, adversely affecting surface quality. Known attempts in the prior art to deal with this deflection problem, however, have not been satisfactory and have not, so far as is known, involved any significant alteration of the conventional tool rise pattern. For example, the U.S. Pat. No. 2,645,980 to Bedker discloses a broach in which the turning workpiece is confined between a broach and a support block, one on either side of the part, with a resilient spring used to absorb the oscillations. Such a structure is just not practical for production for several reasons, including the greatly increased friction that would result from attempting to trap a rapidly spinning turning part between two surfaces, and because of the extra space required Fundamentally, such a design does nothing to deal with the root cause of the problem, which is part deflection; it merely tries to tolerate it.
The deflection problem is also recognized in the Tool and Manufacturing Engineers Handbook, an SME publication, at Chapter 7, page 14, where it is said that the broach finishing teeth may be "back stepped" to compensate for elastic spring back of the workpiece. While the term "back step" is not explained or defined, an accompanying illustration makes it clear what is meant by "back stepping" the finish teeth is that, while they are still in a straight line, the slope of the line is less. That is, the tool rise for the finishing teeth is still uniform or linear, but it is a smaller uniform rise per tooth. In effect, two of the basic tooth patterns described above are placed end to end, but the basic pattern is not altered.